Java – use ‘select’ tags and entities in thymeleaf

I am creating a form with a select tag as follows:

<form th:object="${version}" method="post" class="form-horizontal">
    ...
    <div class="control-group" th:classappend="${#fields.hasErrors('product')} ? 'error'">
        <label class="control-label" for="product" th:text="#{version.product}">Product</label>
        <div class="controls">
            <select id="product" th:field="*{product}">
                <option value="" th:text="#{common.select.prompt}"></option>
                <option th:each="p : ${productList}" th:value="${p.id}"  th:text="${p.name}"></option>
            </select>
            <span class="help-inline" th:errors="*{product}"></span>
        </div>
    </div>
    ...
</form>

The domainclassconverter class of spring data JPA helps me to automatically convert the selected ID to the entity product when I submit the form The product should also not be null (I use @ notnull in the product field of the version class)

My problem – when I came back to edit the data, no product was selected

If I modify the selection like this (th: field and th: errors): < - ps.s Not a sad smile

<select id="product" th:field="*{product.id}">
    <option value="" th:text="#{common.select.prompt}"></option>
    <option th:each="p : ${productList}" th:value="${p.id}" th:text="${p.name}"></option>
</select>
<span class="help-inline" th:errors="*{product.id}"></span>

Then when I come back to edit it, it will be selected, but the verifier will not work (the product is always instantiated, even if the selected ID is null)

It looks like a very common scenario (select an entity from the list), but I can't find any good examples Please share your secret knowledge

Solution

It's settled The problem is that I didn't override the equals () and hashcode () methods